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Testing new strategies to improve the recovery of phosphorus from anaerobicallydigested organic fraction of municipal solid waste

Oliveira, Verónica; DiasFerreira, Celia; Labrincha, João; Rocha, Joana L.; Kirkelund, Gunvor Marie

Published in:Journal of Chemical Technology and Biotechnology

Link to article, DOI:10.1002/jctb.6037

Publication date:2019

Document VersionPeer reviewed version

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Citation (APA):Oliveira, V., DiasFerreira, C., Labrincha, J., Rocha, J. L., & Kirkelund, G. M. (2019). Testing new strategies toimprove the recovery of phosphorus from anaerobically digested organic fraction of municipal solid waste.Journal of Chemical Technology and Biotechnology. https://doi.org/10.1002/jctb.6037

https://doi.org/10.1002/jctb.6037https://orbit.dtu.dk/en/publications/f866ca9c-1db3-4c1f-ab04-df436fc0c313https://doi.org/10.1002/jctb.6037

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/jctb.6037

Testing new strategies to improve the recovery of phosphorus from

anaerobically digested organic fraction of municipal solid waste

Verónica Oliveira1,2*, Celia Dias-Ferreira1,2,3, João Labrincha2, Joana L. Rocha1, Gunvor M.

Kirkelund4

1 Polytechnic Institute of Coimbra & Research Centre for Natural Resources, Environment and Society

(CERNAS), College of Agriculture, Bencanta, 3045-601 Coimbra, Portugal

2 Materials and Ceramic Engineering Department, CICECO, University of Aveiro, Campus

Universitário de Santiago, 3810-193 Aveiro, Portugal

3 Universidade Aberta, Lisboa, Portugal

4 Department of Civil Engineering, Technical University of Denmark, building 118, 2800 Kgs.

Lyngby, Denmark

*corresponding author. Tel.: +351 239 802 600 Fax: +351 239 802 979 E-mail address:

[email protected]

Abstract

BACKGROUND: This work is focused on phosphorus recovery from anaerobically digested organic

fraction of municipal solid waste (referred to as “digestate”) as a fertiliser. The main purpose was to

propose and test modifications to the electrodialytic process that increase phosphorus extraction,

improve the quality of the fertiliser by removing contaminants, and reduce hydraulic retention time to

This article is protected by copyright. All rights reserved.

allow for smaller system footprints. Strategies tested were: i) lowering the pH of the digestate

suspension to below 4.5 using the electrochemical reactions and enhance phosphorus solubilisation

from the waste; ii) change the configuration of the electrodialytic cell from 3- to 2-chambers; and iii)

stir the sample to shorten the duration of the extraction.

RESULTS: Results show that the acidification of digestate by the electrochemical reactions was

effective to enhance phosphorus extraction yield. Three-chamber electrodialytic experiments enabled

the removal of heavy metals from digestate, producing phosphorus-rich solutions with a low level of

metals. This resulted in the production of high quality fertiliser which can be used for agricultural

applications. The modification of electrodialytic cell set-up from 3-chamber to 2-chamber did not

result in an increase of the phosphorus extraction yields nor did it contribute to the removal of metals

from the liquid phase of digestate. The reduction of the hydraulic retention time of electrodialytic

extraction of phosphorus from 16d to 9d was attained by the use of stirring and by the electrodialytic

acidification.

CONCLUSIONS: The increase of phosphorus recovery was accomplished. 90% of phosphorus was

successfully extracted from digestate and transformed into struvite.

Keywords

Electrodialytic process; Heavy metals; Municipal solid waste; Phosphorus; Struvite.

1. Introduction

The decline of phosphate rock reserves used by the fertiliser industry has led to the search of

alternative sources of phosphorus. Several waste streams have been pointed out as potential secondary


sources from where phosphorus can be recovered.1 Municipal solid waste (MSW) contains up to 30 %

- 50 % of organic waste,2 the majority of which food scraps rich in phosphorus (up to 4.0 g kg-1, dry

matter).3 The sludge resulting from the treatment by anaerobic digestion (herein referred to as

“digestate”) of these organic wastes is a potential secondary source of phosphorus, provided no

contaminants are present. Annually, around 56 Mtonnes of digestate are produced in Europe,2 and this

figure will likely increase in the future. Depending on the feedstock to the anaerobic digester, the

digestate has two main final destinations: (i) application in agriculture (usually after composting) due

to its high available fraction of nitrogen and phosphorus; or (ii) landfilling.

Use of digestates in agriculture is only possible when the levels of contaminants are low. Most large

scale anaerobic digestors for MSW handle a mix of different organic material, including mixed and/or

unsorted waste where the levels of contaminants are higher than in the pure fractions. Such digestate

usually fails to meet the criteria for agriculture application and large amounts end up landfilled or are

used in non-agricultural applications where their nutrient content is not valorised.2 In either case, the

nutrients are lost. This practice is contradictory to the circular economy principles, because while the

global reserves of phosphate rock fastly decline, a potential source of phosphorus is not correctly

recycled and/or utilised. This framework inspired the current work, in which the recovery of

phosphorus from digestate is addressed. The main challenge in the valorisation of digestate is the

ability to extract phosphorus while leaving behind the contaminants. Amongst the contaminants, heavy

metals are of special concern due to their potentially negative effects in soils and crops.4

Among extraction techniques, the electrodialytic (ED) process is pointed out as a feasible technique

that has been used to extract phosphorus from a few waste streams: i) wastewater,5 ii) sewage sludge6

and their ashes7–11 and iii) MSW digestate.12 The principle of this technology has been explained

elsewhere.13 Briefly, during the ED process, a low-intensity electric current crosses a water-saturated

waste promoting the transport of the soluble charged ions towards the electrode of opposite charge.


Since anions are attracted to the anode and cations to the cathode, ED promotes the simultaneous

extraction and build-up of anions and cations into separate chambers. In a more detailed analysis, the

extraction of phosphorus using the ED technique involves different processes. In the first stage,

phosphorus dissolves from the solid waste particles into the solution. The phosphorus concentration in

the solution increases until the equilibrium concentration is reached. At that point, dissolution and the

precipitation rates are equal, and the concentration in solution does not change. In the second phase,

dissolved phosphorus ions (negatively-charged) migrate across the water-saturated waste in the

direction of the anode, driven by the electric field. Finally, in the third phase the phosphorus is driven

across the inner pores of the anion-exchange membrane, reaching the anode-side chamber. When

phosphorus is removed from the waste chamber, the dissolution equilibrium is disturbed, and

additional phosphorus dissolves from the waste. This is again followed by migration of dissolved

phosphorus towards anode. The dissolution and transport processes occur continuously, so the

equilibrium is ideally not reached. This mechanism means that ED process can potentially remove

more phosphorus than other extraction methods. Eventually all phosphorus dissolves from the waste

and moves into anode-side chamber.

In a previous work, 42 % of the total phosphorus present in digestate was extracted using the ED

process.12 Extracted phosphorus was subsequently transformed into a struvite-based fertiliser. Even

though this earlier work highlighted the potential of digestate from MSW to serve as a secondary

source of phosphorus, there were still several problems that needed to be tackled, namely the low

extraction yields (more than 50 % of phosphorus in the waste was not recovered), the presence of

heavy metal contaminants in the fertiliser (Cd, Pb, and Cu were eliminated, but Zn levels ranged 0.6 -

1.4 g kg−1), and the high hydraulic retention time required for the extraction (16 d).

The main purpose of the current work is to propose and test several modifications to the ED extraction

in order to improve the recovery of phosphorus from digestate. The targets are three-fold: i) achieve


higher phosphorus extraction yields from this waste; ii) improve the quality of the fertiliser produced

by decreasing the level of contaminants; and iii) reduce hydraulic retention time to allow for smaller

system footprints and lower investment costs.

2. Materials and methods

2.1 Strategies proposed for improving the recovery of phosphorus from digestate

The strategies proposed herein to tackle potential transport-related issues are: a) decrease the distance

phosphorus needs to travel driven by the electric field through stirring of the digestate suspension and;

b) avoid the transport of phosphorus across the anion-exchange membrane by changing the

configuration of the ED cell. The strategy proposed herein to tackle dissolution-related constrains are

to lower the pH within the waste chamber through the electrochemical reactions taking place at to

enhance dissolution of the digestate. Figure 1 summarises the three strategies that were applied in the

present work according to the specific targets established.

(Fig. 1 here)

2.2 Testing the strategies

2.2.1 Digestate

Digestate was collected from an integrated center for treatment and recovery of municipal solid waste

located in Portugal. In this facility, the organic fraction of municipal solid waste is firstly mechanically

separated from mixed waste, then submitted to anaerobic digestion and finally composted. The sample


was collected after centrifugation of the sludge from the anaerobic digester. The sample was then

transported to the laboratory under refrigerated conditions (< 4 ºC), dried upon arrival (105 ºC, 24 h),

grounded (< 1 mm) and stored in closed containers until use in extraction experiments.

The characteristics of digestate are shown in Table 1. Digestate presented a slightly alkaline pH and a

total phosphorus content of about 0.8 %.

(Table 1 here)

2.2.2 ED experiments to extract phosphorus from digestate

Two different set-ups (depicted in Fig. 2) were used to extract phosphorus from digestate. The 3-

chamber set-up (Fig. 2a) aimed at moving phosphorus out from waste chamber into anode-side

chamber and at moving metal cations out from waste chamber into cathode-side chamber. The 2-

chamber set-up (Fig. 2b) aimed at moving metal cations out of waste chamber into cathode-side

chamber, leaving solubilised phosphorus in waste chamber.

(Fig. 2 here)

The ED cells used were cylindrical, made of Plexigas and with an internal diameter of 8 cm. Waste

chamber had a length of 10 cm and contained a suspension of 35 g of digestate in 350 mL of distilled

water (L:S = 10 mL g-1). The anode- and cathode-side chambers were 5 cm long and contained

electrolyte solutions of 0.01 M NaNO3 (adjusted to pH < 2 with HNO3).


Electrodes consisted of rods of platinum coated titanium, 3 mm in diameter and 5 cm long (from

Permascand). In the 3-chamber set-up the cathode was placed in the cathode-side chamber and the

anode in the anode-side chamber. In the 2-chamber set-up the cathode was placed in the cathode-side

chamber (as in the 3-chamber set-up) but the anode was placed directly in the waste chamber.

Plastomec pumps were used to recircule the electrolytes between the electrode compartments and

glass flasks, to eliminate the gases released during the electrode reactions and avoid the formation of a

layer with a high concentration of ions close the membranes.14

A physical separation between waste chamber and the cathode-side chamber was made using a cation

exchange membrane (CR67 HUY N12116B) and only for the 3-chamber set-up an anion exchange

membrane (204 SZRA B02249C) was used to separate the waste chamber from the anode-side

chamber.

Eight ED experiments were carried out. Four using the 3-chamber set-up with a duration of 3, 7, 9 and

11 d and the other four using the 2-chamber set-up for the same period. In all experiments, a constant

current intensity of 50 mA was applied (Hewlett Packard E3612A) and the suspension of digestate was

continuously mixed using a VWR VOS 14 stirrer (H 500 rpm).

During the ED experiments, pH and electric conductivity were measured daily in the digestate

suspension by electrodes. pH was also measured in the cathode-side chamber since its pH value should

be maintained below 2 in order to avoid the formation of metal hydroxides precipitates nearby the

cation exchange membrane (which would hinder ion transport through the ED cell).14 A record of the

current intensity and voltage drop across the cell was done. In addition, some samples were collected

from waste chamber when an alteration of waste suspension pH was noted, in order to determine

phosphorus speciation during ED process.

At the end of the ED experiments, the volume of electrolytes was recorded and the digestate


suspension was filtered using quantitative filter paper. The volume of liquid phase of digestate was

registered and the solid phase was dried in an oven at 105 ºC, overnight. Ion exchange membranes and

electrodes were immersed in 1 M and 5 M HNO3, respectively, during 24 h, to release any elements

attached to them. Contents of phosphorus, calcium, magnesium, iron, aluminium and heavy metals

(Cd, Cr, Cu, Ni, Pb and Zn) were quantified in solid (after its crushing in a mortar) and liquid phase of

digestate and in filtered (0.45 µm syringe filter) electrodes and ion exchange membranes cleaning

solutions, according to the procedure described in section 2.5.

2.2.3 Phosphorus extraction yield

In the 3-chamber experiments, the phosphorus extraction yield was calculated as the mass of

phosphorus found in the anolyte at the end of experiment divided by the total mass of phosphorus

found in the different parts of ED cell at the end of the experiment. In the 2-chamber experiments, the

phosphorus extraction yield was based in the relation between the mass of phosphorus found in liquid

phase of digestate and the total mass found in different parts of ED cell at the end of the experiment.

2.3 Modelling of phosphorus speciation during 2-chamber ED experiments

To evaluate the phosphorus speciation during 2-chamber ED experiments, the data of chemical

composition and pH of the liquid phase of digestate was used in the software Visual Minteq 3.1. The

calculations assumed a temperature of 25 ºC and the ionic strength was calculated by the software.


2.4 Phosphorus transformation into struvite

Chemical precipitation of struvite was made in order to promote the recovery of phosphorus from the

digestate into a solid form that can be used for agriculture applications. The anolyte solutions from the

3-chamber experiments and the liquid phase of digestate solutions from the 2-chamber experiments,

with duration of 7, 9 and 11 d, were used for phosphorus precipitation as struvite, following the

procedure described in Oliveira et al.12 Briefly, while the solutions were stirring, it was added

MgCl2·6H2O and NH4Cl in a molar ratio Mg:N:P of 2:2:1, and the pH was raised to 9 using 5 M

NaOH. After 60 min of reaction, the struvite-suspension was filtered through 0.45 µm membrane filter

and the struvite-precipitates were dried at 50 °C for 48 h. The precipitates obtained were analysed by

X-ray diffraction (XRD, Rigaku Geigerflex (JP) with a Cu anode, operating at 20 kV and 40 mA) and

scanning electron microscopy coupled with energy dispersion spectroscopy (SEM-EDS) using Hitachi

SU-70 equipped with EDS – Bruker. The contents of phosphorus, calcium, magnesium, iron,

aluminium and heavy metals were analysed in the precipitates.

2.5 Analytical methods

The digestate’s pH and electric conductivity were measured using Radiometer electrodes after 1 h or

30 min agitation, respectively, of a suspension of 5.0 g of dry sample in 12.5 mL of distilled water.

Total contents of phosphorus, calcium, magnesium, iron, aluminium and heavy metals in digestate,

solid samples from ED experiments and struvite precipitates were measured in triplicate after a pre-

treatment described in Danish Standard DS259: 1.0 g of dry sample was placed into a schott flask and

20.0 mL of 1:1 HNO3 was added. The flasks were heated in an autoclave at 200 kPa (120 ºC) during

30 min. Then the samples were filtered by vacuum through a 0.45 µm filter and diluted to 50 mL. A

Varian 720-ES ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry) was used to


quantify the elemental concentration in pre-treated samples as well as in all samples from ED and

precipitation experiments.

3. Results and discussion

3.1 Phosphorus extraction yields

3.1.1 Development of pH during the ED process

Low extraction yields are related to either the dissolution phase or to the transport phases. The low

dissolution of phosphorus compounds, especially at the natural pH of the digestate (slightly alkaline,

7.7 - 7.8) can be addressed by acidification of the digestate. In the current work, the strategy proposed

was to enhance solubilisation by lowering the pH of the digestate suspension to below 4.5, using the

electrochemical reactions occurring during ED. The effect of the electrochemical reactions on the

acidification of digestate suspension was assessed by measuring pH in the digestate suspension during

the extraction (Fig. 3). At the beginning of the experiments, the pH in the digestate suspensions ranged

7.4 - 8.1. During the experiments, the pH decreased, and the decrease was faster during the 2-chamber

experiments than during the 3-chamber experiments: a pH 3 - 3.5 was registered after 3 d in the 2-

chamber, while in the 3-chamber similar pH values were only reached after 6 d or 7 d. In addition, for

the same experimental time, the pH of the digestate suspension was 1-3 units below in 2-chamber

experiments, when compared to the 3-chamber experiments. Acidification in the 2-chamber ED cell

occurs due to the generation of H+ by the half-reaction taking place at the anode: 2𝐻2𝑂 → 𝑂2(𝑔) +

4𝐻+ + 4𝑒− (Eq. 1). Whereas, in the 3-chamber experiments, the same reaction takes place at the

surface of the anion-exchange membrane (phenomena denominated as water splitting15,16) separating

waste chamber and anode-side chamber, leading to acidification of the waste inside waste chamber.

This explains the different acidification rates. In both ED cell set-ups, acidification of the digestate


suspension in the waste compartment was successfully achieved by the electrochemical reactions

alone, without the need for adding external acids. Overall, an identical development of pH was

observed over time in all experiments which indicates a good repeatability and robustness of the ED

experiments. In the 2-chamber experiments, the decrease in pH was accompanied by a progressive

increase of the electric conductivity (Fig. 3). This was mainly due to the solubilisation of ions from the

digestate and also to the addition of H+ ions directly into the waste compartment due to water

electrolysis at the anode (Eq. 1). Oppositely, in the 3-chamber experiments, a decrease of the electric

conductivity in the digestate suspension was registered. During the first 3 d, the electric conductivity

decreased drastically and then it remained constantly low, at around 0.4 mS cm-1, until the end of ED

process. This is explained by the electromigration of the dissolved anions, such as phosphorus, out of

waste chamber into the anode-side chamber and of cations, such as heavy metals into the cathode-side

chamber.

(Fig. 3 here)

3.1.2 Comparison of 2- and 3-chamber ED cell

Figure 4 shows the distribution of phosphorus (in percentage) in the ED cell at the end of the

experiments. Overall, in the 2-chamber experiments phosphorus was mostly (86 %) in the waste

chamber (liquid phase: 70 %; solid phase 16 %), while a smaller percentage (14 %) was in the cathode

chamber. Oppositely, in the 3-chamber experiments, less than 3 % of phosphorus was found in the

liquid phase of digestate, and most phosphorus in solution was transported out of waste chamber into

the anode-side chamber (89 %), as intended (to separate phosphorus from the digestate). In both set-

ups a residual amount of phosphorus (16 % in the 2-chamber and 9 % in the 3-chamber) was not


dissolved during the extraction and remained in the solid phase of the original material.

Solubilisation of phosphorus occurred since the very beginning of the ED process, with more than half

the phosphorus being extracted in the first 3 d: 54 % in the 3-chamber experiment (at pH = 5.8) and

58 % in the 2-chamber experiment (at pH = 2.8). For both 2- and 3-chamber experiments, the

phosphorus extraction yields increased as the pH decreased.

(Fig. 4 here)

The extraction of phosphorus did not evolve after 9 d, with the values for 9 d and 11 d being identical.

Overall, after 9 d a higher amount of phosphorus was extracted from the digestate in the 3-chamber

experiments (90 %) than in the 2-chamber experiments (71 %). This result was not anticipated, since

in other studies comparing both set-ups (to extract phosphorus from wastes), the 2-chamber performed

better.8,17

A pH range of 2.5 - 3.0 seems to be favourable to extract phosphorus from the digestate using a 3-

chamber ED cell (Fig. 4b). In a 2-chamber ED cell, a lower pH 1 - 2 favoured the extraction. A

suitable pH range of 2.5 - 3.0 was attained after 7 d in the 3-chamber experiments; however,

phosphorus continued to be extracted until day 9. This indicates that the phosphorus extraction from

digestate did not occur immediately and there is a gap between the time an adequate pH range for the

extraction is reached and the time when no more phosphorus is extracted.

Based on the results above, lowering the pH of digestate suspension to below 4.5 (previously found in

Oliveira et al.12) by acidification of the digestate suspension using the electrochemical reactions is an

important contributor to the enhancement of phosphorus solubilisation, which consequently increases

phosphorus extraction yields.


A relevant detail for the 2-chamber experiment is that phosphorus started to appear in the cathode-side

chamber after 7 d, reaching 14 % of total phosphorus after 11 d (Fig. 4a). This result indicates that

prolonging the 2-chamber experiments after the time where the maximum phosphorus extraction yield

was achieved (7 d) meant a higher loss of phosphorus into the cathode-side chamber. Oppositely, for

the 3-chamber experiments phosphorus was not transported to the cathode-side chamber (less than

0.2 %).

3.2 Distribution of heavy metals at the end of ED process

Figure 5 presents the percentage of metals found in the different parts of the ED cell at the end of the

extraction process. The cathode-side chamber included the catholyte, the cathode and the cation

exchange membrane. In the 3-chamber experiments, the anolyte, the anode and the anion exchange

membrane were grouped in the anode-side chamber.

Precipitation of struvite is induced from different chambers, depending on the cell set-up. In the 3-

chamber experiments, struvite is obtained from the anode-side chamber and in the 2-chamber

experiment from the digestate suspension chamber. From the perspective of reducing the presence of

potential contaminants in struvite it is important that these chambers do not contain heavy metals in

solution at the end of the extraction step.

Overall, 70 % to 90 % of Zn and more than 40 % of Cd and Ni were removed to the cathode-side

chamber. Cu, Cr and Pb mainly remained in the solid phase of the digestate (more than 60 %).

Regarding the presence of metals in the solutions that were further used to produce struvite, between

10 % and 20 % of the metals were found in the liquid phase in the 2-chamber experiments. In the 3-

chamber experiments only a residual amount of metals (less than 4 %) were found in the anode-side

chamber. Furthermore, the presence of Zn in the anode-side chamber (seen in Oliveira et al.12) was


avoided in the current work. The reduction of the experimental time from 16 d (previous reported

results,12) to 11 d, 9 d or 7 d (current work) and the final pH of digestate suspension from about 4.5

(previous reported results12) to about 2.5 - 3.0 may explain the low mobilisation of Zn into the anode-

side chamber.

(Fig. 5 here)

3.3 Modelling phosphorus speciation

The unwanted transport of phosphorus into the cathode-side chamber seen during the 2-chamber

experiments was not ideal, because all the metals cations that are contaminants in the fertiliser are also

removed into this chamber. Phosphorus in the cathode-side chamber is therefore considered to be lost

from the point of view of producing a fertiliser. Transport of phosphorus into the cathode-side occurs

if positively-charged phosphorus species are formed. So, a deeper analysis was carried out to

understand how the other elements present in the waste suspension affect the formation of positively

charged phosphorus-species, using the modelling software MINTEQ. This analysis was not performed

for the 3-chamber experiments because the presence of phosphorus in the cathode-side chamber was

not observed and the charged phosphorus ions were mostly moved into the anode-side chamber. Table

2 shows the probable speciation of phosphorus at different pH values considering the chemical species

measured in the waste suspension during the 2-chamber experiments. At lower pH values (< 3), in

addition to the existence of negative (H2PO42-) and neutral (H3PO4) phosphorus forms, occurs also the

complexation of phosphorus with Fe3+, Ca2+ and Al3+ in solution, forming positively-charged species:

FeHPO4+, FeH2PO4

+2, CaH2PO4+ and AlHPO4

+. The presence of these positively-charged forms of

phosphorus explains the phosphorus found in the cathode-side chamber.


(Table 2 here)

3.4 Evaluation of struvite quality

3.4.1 Structure and elemental composition of struvite

Figure 6 shows the XRD pattern of the precipitates harvested during the struvite-production stage. For

the 3-chamber experiments, a crystalline phase showed up, matching the profile (position and intensity

of the peaks) of the struvite standard. No extra/minor phases were identified in the XRD pattern,

consistent with a precipitate of pure struvite. XRD results for the precipitates harvested from the 2-

chamber experiments indicates that there were no crystalline phases for the 7 d and 11 d experiments,

so struvite crystals were not formed in these two experiments. In the 9 d-experiment, there was a

crystalline phase consistent with struvite; however, there was background noise affecting the peak

definition, indicating that impurities were present. Le Corre et al.18 reported that as the calcium

concentration in solution increases, the struvite formation can be hindered, either due to calcium

competing for the phosphate ions or because this element interferes with the crystallisation of struvite.

There were large amounts of calcium ions present in the solutions the 2-chamber experiments

(Table 3), especially for 7 d and for 11 d, explaining why struvite formation was strongly hindered in

these two experiments. In the 3-chamber experiments, there was almost no calcium in solution, likely

because this cation has been moved towards the cathode-side chamber.

(Table 3 here)


The EDS spectrums of the precipitates are shown in Fig. 7. The major elements were magnesium and

phosphorus, consistent with struvite. For the case of 2-chamber experiments, a distinctive peak for

calcium was also identified, while struvite peaks were still detectable. The calcium peak identified on

precipitates obtained from the 7 d and 11 d- liquid phase solutions was higher than the obtained at 9 d,

which likely led to the formation of an amorphous substance (such as calcium phosphates) in place of

struvite (Fig. 6a: 7 d and 11 d).

(Fig. 6 here)

(Fig. 7 here)

3.4.2 Assessment of the struvite fertiliser for application in agriculture

Up to 98 % of phosphorus in solution after the extraction stage was recovered in solid form in the

struvite-inducing experiments. The percentage of phosphorus in the precipitates obtained from the 3-

chamber experiments is almost twice what was measured in the precipitates from the 2-chamber

experiments (Table 4). Additionally, the precipitates obtained from 2-chamber experiments contained

levels of Pb, Cd and Zn above the Portuguese legal limits for fertilisers, which means they cannot be

used in agriculture. Contrarily, the precipitates obtained from the 3-chamber experiments contained

low level of metals (Cr, Pb and Zn), which did not exceed the limits of metal contents established by

the Portuguese legislation for fertilisers (Table 4), which highlights their lower level of contaminants


and superior quality.

(Table 4 here)

3.5 Hydraulic retention time of ED extraction of phosphorus

The utilisation of a stirrer in the waste chamber and the acidification of digestate suspension below pH

4.5 were the strategies proposed to reduce the hydraulic retention times. In a previous work (Oliveira

et al.12) it took 16 d to extract about 130 mg of phosphorus using a 3-chamber ED cell identical to the

one used in the current work, but in a stationary set-up (without stirring). Table 5 compares the amount

of phosphorus extracted in the previous and in the current work, normalised for the electric charges

passed during the experiments (expressed in Coulomb, C).

(Table 5 here)

A similar amount of phosphorus was found at the end of the 3-day stirred ED experiment and the

stationary experiment; however, twice the amount of phosphorus was removed per unit of Coulomb,

and only 3 d were necessary, instead of 16 d. This result demonstrates the positive influence of the

utilisation of a stirrer on the reduction of the hydraulic retention time of ED process. The other stirred

ED experiments (7, 9 and 11 d) showed that as the extraction time proceeded it became ever more

difficult to extract the remaining phosphorus, since the amount of phosphorus extracted per unit charge

passing through the cell decreases. Eventually the amount phosphorus extracted is kept constant, even

though electric charges are still being passed through the cell, as was seen between 9 d and 11 d. These


results reveal that the hydraulic retention time of ED extraction of phosphorus from digestate can be

reduced by 44 % (from 16 d to 9 d).

4. Conclusions

In the present work, the effects of modifications to the ED process to improve the recovery of

phosphorus from anaerobically digested organic fraction of municipal solid waste were evaluated.

Under the experimental conditions tested here, the following conclusions were drawn:

i) the acidification of digestate to a pH below 4.5 using the electrochemical reactions occurred

during ED process led to an enhancement of phosphorus extraction yield.

ii) the ED extraction of phosphorus from digestate is higher for the 3-chamber ED cell set-up.

The proposed strategy of changing the configuration of the ED cell to 2-chamber did not

contribute to the increasing of phosphorus extraction yield.

iii) A low presence of heavy metals in phosphorus rich solutions from the 3-chamber ED

experiments, results in the production of a high quality struvite that complies with regulatory

limit values to be used as fertiliser in agriculture. Opposedely, in the 2-chamber ED

experiments, the ED process was not effective for removing heavy metals and a contaminated

material was produced, that failed the limit values for agricultural application.

iv) the utilisation of a stirrer in the waste compartment and the acidification of digestate promoted

by the electrochemical reactions were important contributors to the reduction of the hydraulic

retention time of ED extraction of phosphorus.

Overall, the objectives proposed in the current work were accomplished: i) the phosphorus extraction

yield from digestate was considerably enhanced; ii) the harvested precipitates have high quality to be

applied as fertiliser and; iii) the hydraulic retention time of ED extraction of phosphorus was


substantially reduced.

Acknowledgements

The authors would like to thank Sabrina Hvid for carrying out ICP analyses. This work has been

funded by Portuguese National Funds through FCT – Portuguese Foundation for Science and

Technology under CERNAS (UID/AMB/00681/2013). Celia Dias-Ferreira and Verónica Oliveira have

been funded through FCT “Fundação para a Ciência e para a Tecnologia” by POCH – Programa

Operacional Capital Humano within ESF – European Social Fund and by national funds from MCTES

(SFRH/BPD/100717/2014; SFRH/BD/115312/2016). Joana L. Rocha has been funded by project

0340-SYMBIOSIS-3-E co-funded by FEDER “Fundo Europeu de Desenvolvimento Regional”

through Interreg V-A España-Portugal (POCTEP) 2014-2020.

References

1. Karunanithi R, Szogi AA, Bolan N, Naidu R, Loganathan P, Hunt PG, Vanotti MB, Saint CP,

Ok YS and Krishnamoorthy S, Phosphorus recovery and reuse from waste streams. Adv. Agron.

131: 173–250 (2015).

2. European Commission. End-of-waste criteria for Biodegradable waste subjected to biological

treatment (compost & digestate): Technical proposals Final Report December 2013. (2014).

3. Kalmykova Y and Fedje KK, Phosphorus recovery from municipal solid waste incineration fly

ash. Waste Manag. 33: 1403–1410 (2013).

4. Singh J and Kalamdhad AS, Effects of heavy metals on soil, plants, human health and aquatic

life. Int. J. Res. Chem. Environ. 1: 15–21 (2011).

5. Ebbers B, Ottosen LM and Jensen PE, Electrodialytic treatment of municipal wastewater and


sludge for the removal of heavy metals and recovery of phosphorus. Electrochim. Acta 181:

90–99 (2015).

6. Guedes P, Magro C, Couto N, Mosca A and Mateus EP, Potential of the electrodialytic process

for emerging organic contaminants remediation and phosphorus separation from sewage

sludge. Electrochim. Acta 181: 109–117 (2015).

7. Guedes P, Mateus EP, Almeida J, Ferreira AR, Couto N and Ribeiro AB, Electrodialytic

treatment of sewage sludge: Current intensity influence on phosphorus recovery and organic

contaminants removal. Chem. Eng. J. 306: 1058–1066 (2016).

8. Guedes P, Couto N, Ottosen LM, Kirkelund GM and Mateus E, Valorisation of ferric sewage

sludge ashes: Potential as a phosphorus source. Waste Manag. 52: 193–201 (2016).

9. Guedes P, Couto N, Ottosen LM and Ribeiro AB, Phosphorus recovery from sewage sludge

ash through an electrodialytic process. Waste Manag. 34: 886–892 (2014).

10. Viader RP, Jensen PE, Ottosen LM, Ahrenfeldt J and Hauggaard-Nielsen H, Electrodialytic

extraction of phosphorus from ash of low-temperature gasification of sewage sludge.

Electrochim. Acta 181: 100–108 (2015).

11. Viader RP, Jensen PE, Ottosen LM, Thomsen TP, Ahrenfeldt J and Hauggard-Nielsen H,

Comparison of phosphorus recovery from incineration and gasification sewage sludge ash.

Water Sci. Technol. 75: 1251–1260 (2017).

12. Oliveira V, Labrincha J and Dias-Ferreira C, Extraction of phosphorus and struvite production

from the anaerobically digested of organic fraction of municipal solid waste. J. Environ. Chem.

Eng. 6: 2837–2845 (2018).

13. Guedes P, Couto N, Mateus EP and Ribeiro AB, Phosphorus Recovery in Sewage Sludge by

Electrokinetic Based Technologies: A Multivariate and Circular Economy View. Waste and

Biomass Valor 8: 1587–1596 (2017).


14. Ebbers B, Ottosen LM and Jensen PE, Comparison of two different electrodialytic cells for

separation of phosphorus and heavy metals from sewage sludge ash. Chemosphere 125: 122–

129 (2014).

15. Nystroem GM, Ottosen LM and Villumsen A, Acidification of Harbor Sediment and Removal

of Heavy Metals Induced by Water Splitting in Electrodialytic Remediation. Sep. Sci. Technol.

40: 2245–2264 (2005).

16. Ottosen LM, Hansen HK and Hansen CB, Water splitting at ion-exchange membranes and

potential diferences in soil during electrodialytic soil remediation. J. Appl. Electrochem. 30:

1199–1207 (2000).

17. Ebbers B, Ottosen LM and Jensen PE, Comparison of two different electrodialytic cells for

separation of phosphorus and heavy metals from sewage sludge ash. Chemosphere 125: 122–

129 (2015).

18. Le Corre KS, Valsami-Jones E, Hobbs P and Parsons SA, Impact of calcium on struvite crystal

size, shape and purity. J. Cryst. Growth 283: 514–522 (2005).

19. Ministério da Economia. Decreto-Lei no103/2015 de 15 de Junho. Diário da República, 1a

série - No 114 (In Portuguese). pp 3756–3788 (2015).


Table 1 Characteristics of digestate (average ± standard deviation).

Characteristic Value

pH 7.67 – 7.71

Conductivity (mS cm-1) 5.72 ± 0.39

Total elements

P (g kg-1) 7.53 ± 0.48

Ca (g kg-1) 70.79 ± 3.31

Mg (g kg-1) 10.94 ± 0.78

Al (g kg-1) 8.16 ± 0.56

Fe (g kg-1) 7.38 ± 0.40

Zn (mg kg-1) 351.35 ± 74.39

Pb (mg kg-1) 158.57 ± 14.58

Cu (mg kg-1) 148.13 ± 21.90

Cr (mg kg-1) 34.01 ± 2.06

Ni (mg kg-1) 29.30 ± 3.56

Cd (mg kg-1) 1.22 ± 0.05


Table 2 Phosphate speciation (%) in the liquid phase of digestate during the 2-chamber experiments obtained from MINTEQ

software (* real/measured values).

pH* [P]*

mg L-1 H3PO4 HPO4

2- H2PO4- FeH2PO4

+2 FeHPO4+ CaH2PO4

+ AlHPO4+

1.2 744 68 – 10 18 4 0.8 0.1

1.3 734 65 – 11 18 5 0.9 0.2

1.4 738 61 – 16 15 5 3 0.4

1.4 665 58 – 14 19 6 2 0.2

1.5 650 56 – 16 18 7 3 0.3

1.6 785 55 – 19 15 7 2 0.6

1.8 642 51 – 27 10 9 3 1

1.9 700 41 – 31 9 13 2 3

2.8 506 13 – 67 0.3 3 11 6

2.9 484 10 – 70 0.2 3 11 6

3.4 435 4 0.02 70 0.1 4 12 9

3.6 390 2 0.04 79 0.02 1 14 3

4.5 286 0.3 0.3 82 – 0.7 14 1

5.0 194 0.1 0.9 84 – 0.9 11 1

5.3 181 0.1 2 83 – 0.8 10 2

5.4 192 0.04 2 81 – 1 10 1

5.7 184 0.02 4 79 – 1 8 1

7.4 31 – 49 21 – 0.02 0.4 –


Table 3 Concentration of calcium in the solutions where struvite precipitation was promoted (mg L-1) obtained at the end of

2- and 3-chamber ED experiments with duration of 7 d, 9 d and 11d.

Element 2-chamber 3-chamber

7 d 9 d 11 d 7 d 9 d 11 d

Ca (mg L-1) 600.8 271.6 641.3 1.4 3.2 3.7


Table 4 Contents of phosphorus and heavy metals in the harvested precipitates from 2- and 3-chamber ED experiments with

duration of 7, 9 and 11 d (average ± standard deviation). In bold are the values outside the Portuguese legislation limits for

application in agriculture (*Insufficient sample for replicates; ND: not detected).

Elements

ED cell set-up / duration Portuguese limits for

fertiliser

application19

2-chamber /

7 d

2-chamber

/ 9 d*

2-chamber

/ 11 d

3-chamber

/ 7 d

3-chamber

/ 9 d

3-chamber

/ 11 d

P (g kg-1) 101.3±2.2 103.6 101.9±0.3 196.3±3.8 171.3±6.2 163.2±0.6 -

Cd (mg kg-1) 2.4±0.05 3.0

Table 5 Comparison of phosphorus extraction per unit of Coulomb in stationary and stirred ED experiments (* reported in

Oliveira et al.12).

ED cell Duration

(d)

Phosphorus

extracted

(mg)

Phosphorus removed per

Coulomb

(mg C-1)

Phosphorus

extraction yield

(%)

Stationary (*) 16 130 0.0094 43

Stirred 3 142 0.0183 54

7 215 0.0118 82

9 236 0.0101 90

11 233 0.0082 89


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Fig. 1 Summary of proposed strategies to improve phosphorus recovery from digestate


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Fig. 2 ED cell set-ups used for the phosphorus extraction from digestate: a) 3-chamber set-up and b)

2-chamber set-up (AEM - anion exchange membrane and CEM – cation exchange membrane)


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Fig. 3 pH and electric conductivity measured in digestate suspension in a) 2-chamber and b) 3-

chamber ED experiments during 3, 7, 9 and 11 d


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Fig. 4 Percentage of phosphorus found in anolyte, liquid and solid phase of digestate and cathode-

side chamber at the end of a) the 2-chamber experiments and b) the 3-chamber experiments

a) b) pH 1 - 2 pH 3 pH 5.5 pH 3.5 pH 2.5 - 3


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Fig. 5 Distribution of heavy metals (%) in different parts of ED cell at the end of 2-chamber and 3-

chamber experiments, after a) 7 d, b) 9 d and c) 11 d


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Fig. 6 X-ray diffraction of the precipitates obtained after struvite-inducing reaction from the 2- and 3-

chamber experiments after a) 7 d, b) 9 d and c) 11 d

a)

b)

c)

2-chamber 3-chamber


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Fig. 7 Spectra obtained from EDS of the harvested precipitates from a) liquid phase solutions resulted from 2-chamber experiments and b) anolyte solutions

from the 3 chamber experiments, after 7 d, 9 d and 11 d. SEM pictures are inset in EDS spectra


testing new strategies to improve the recovery of phosphorus ......2.1 strategies proposed for...

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